Filtering system

ABSTRACT

A filtering system comprising an inlet ( 11 ) for liquid to be filtered and an outlet ( 9 ) for filtered liquid, as well as filtering means, which system is characterized in that the filtering means, placed in series comprise: a first filter ( 2 ) comprising activated carbon with the capability to also remove particles that are larger than approximately 0.5 μm; an ultrafiltration membrane ( 5 ) for the removal of particles that are larger than approximately 0.02 μm; a post-treatment filter ( 14 ) of activated carbon, optionally provided with a microfiltration membrane ( 13 ), wherein the system also comprises a vessel ( 6 ) for filtered water located between the ultrafiltration membrane ( 5 ) and the post-treatment filter ( 14 ), with the option of using the filtered water from the vessel for back-flushing the ultrafiltration membrane ( 5 ).

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. national phase application of International ApplicationNo. PCT/NL02/00177, filed Mar. 18, 2002, which claims the benefit ofDutch patent application No. 1017681, filed Mar. 23, 2001.

The present invention relates to a filtering system, in accordance withthe preamble of claim 1. The invention also relates to a method ofcleaning an ultrafiltration membrane module, which is part of thefiltering system according to the invention.

A filtering system as mentioned in the preamble of claim 1 is known fromPatent Abstracts of Japan, Vol. 2000, no. 2, 29 Feb. 2000 and JP11.319827 A. Said known filtering system yields drinking watercontaining minerals and prevents backgrowth of bacteria from the outlet.

However, said known filtering system has a disadvantage in that the useof a nanofiltration membrane only provides a low flow rate through thefilter. Therefore, much of the feed water is wasted in this process.Also, the water becomes stagnant in the module. Since inline cleaning istechnically not possible in said known filtering system, bacteria andviruses may accumulate in the module, multiply and eventually pass teakspots in the membrane and the module connectors and contaminate thevessel for storage of filtered water.

The object of the present invention is to provide an improved system inwhich the above-mentioned drawbacks are eliminated. It is a particularobject of the invention to provide a system in which drinking water maybe produced without losing a great deal of water.

The present invention provides the advantage that the use of anultrafiltration membrane makes a larger flow rate through the filterpossible. Furthermore, so as to provide for a cleaning of saidultrafiltration membrane, the valve, which is provided between the firstfilter and the ultrafiltration membrane makes it possible that theultrafiltration membrane is backwashed. Filtered water which is storedin the vessel flows back through the ultrafiltration membranes andremoves any contamination which has been held back by theultrafiltration membrane, after which said contaminants, together withthe water, may be drained through the said valve and discarded.

The advantage to be obtained with the filter system of the presentinvention is that the ultrafiltration go membrane may be kept clean bybackwashing said ultrafiltration membrane with sterile water from thevessel. Therefore, the ultrafiltration membrane does not have to bedismantled so as to have it cleaned.

Therefore, according to another aspect of the invention, it also relatesto a method of cleaning the ultrafiltration membrane module, which ispart of a system according to the invention, in accordance with claim 8.

As a matter of fact, reference is made to JP 11 309447 A (PatentAbstracts of Japan, Vol. 2000, no. 2, 29 Feb. 2000) in which anultrafiltration membrane is mentioned, in the form of a hollow fibremembrane filter, capable of passing all minerals without any water loss.However, said reference does not mention a valve, which makes itpossible to clean an ultrafiltration membrane by means of backwashing.

Also, reference is made to JP 60.068093 A (Patent Abstracts of Japan,Vol. 009, no. 200, 16 Aug. 1985), which mentions a method for filteringwater, using an ultrafiltration membrane. However, said publication doesnot mention a valve according to the present invention, with which it ispossible to clean the ultrafiltration membrane by means of backwashing.

Finally, reference is made to WO 01.07151 A (in the name of Prime WaterSystems GmbH), which describes the use of an activated carbon filter,which is capable of removing particles that are larger then 0.5 μm, as apre-filter to an ultrafiltration hollow fibre membrane filter. However,said publication does not mention the features of the present invention,such as the vessel for the storage of filtered water and the valve formaking it possible to clean the ultrafiltration membrane by means ofbackwashing.

The advantage of the valve in the present invention is that the systemcan be used for filtering liquid when the valve is in the firstposition. When the valve is in the second position, filtered liquid canbe flushed back from the vessel through the ultrafiltration membrane, inorder to remove accumulated contamination from the pores. Via the valve,which is in the second position, the flushed-back filtered liquid (alsoindicated as back wash water) together with the contamination cansubsequently be discharged to the drain.

The ultrafiltration membrane and the post-treatment filter may beinterconnected by means of, for example, a tubing. In this tubing aconnection may be provided for the vessel for filtered water. Thisconnection for the vessel may consist of a branching leading from thisconnecting tubing.

As mentioned above, the ultrafiltration membrane is incorporated in amodule. This ultrafiltration module preferably consists of a bundle ofhydrophilic capillary membranes having an asymmetrical pore structurewith the filtering layer on the inside of the capillary, as a result ofwhich they have a very low flow resistance, and consequently a lowpressure drop over the membranes. According to a preferred embodiment,the membranes are closed at one end and open at the other end, the spacebetween the capillary membranes and the module wall being filled withepoxy resin. The water flows through the perforated wall of the moduleand the membrane wall to the lumen of the capillaries, and is thusfiltered. Via the post-treatment filter the filtered water subsequentlyflows to the outlet or is fed to the vessel for filtered water. Such adead-end ultrafiltration module is described, for example, in theAmerican patent U.S. Pat. No. 5,895,573.

Another embodiment of the ultrafiltration module is one wherein themembranes are open at both ends, and wherein a valve is provided at oneside through which the flow passes from the inside to the outside andwherein the filtered water exits through the side wall of the module.Such an embodiment may be called dead-end flush. The use of such a deadend flush module makes the above-mentioned method of cleaning theultrafiltration membrane considerably more complicated.

If the ultrafiltration membrane has a pore size of approximately 0.02μm, all the particles that are larger than this pore size will beretained. Among others, such a membrane will retain microorganisms suchas bacteria and viruses, resulting in sterilised filtered water.Compounds that are dissolved in the water, such as minerals, will passthrough the membrane. The original concentration of minerals is thusmaintained. Due to the fact that the water is continuously swirlingaround the ultrafiltration module, there will be no stagnation in themodule and the housing so that the risk of germ growth is minimised.

The first filter may be formed, for example, from a felt-like materialin combination with activated carbon incorporated in the felt that isformed preferably by moulding. A favourable property of such a firstfilter module is that it does not, or only very slightly shed carbon,which might clog the ultrafiltration membrane. Moreover, this firstfilter of activated carbon provides a preliminary filtration up toapproximately 0.5 μm. A final advantage of this first filter ofactivated carbon is that the flow resistance and consequently thepressure drop over the block will be reduced compared with other typesof activated carbon filters.

In order to prevent the block of activated carbon from becoming cloggedwhen the liquid to be filtered is more seriously contaminated withrelatively large particles, and to prolong the life of the filteringdevice, the first filter comprises according to a preferred embodiment afirst filtering step provided around the block of activated carbon,forming a supplementary filtering step before the block of activatedcarbon.

This first filtering step may be formed, for example, from a felt-likefilter, optionally folded in concertina fashion around the block ofactivated carbon. These strips of such a first filtering step that areformed by the folds, possess a large filtering surface having a veryconsiderable filtering capacity.

Such a filter is incorporated, for example, in a “801 series filtercartridge”, available from Harmsco Industrial Filters of North PalmBeach, Fla., USA. A moulded block of activated carbon as mentioned abovemay consist of, for example, a “C-246 series moulded block filtercartridge”, available from Fibredyne, of Dover, N.H., USA.

The back wash provision essentially consists of the valve between thefirst filter and the ultrafiltration membrane module. When said valve isin the second position, water will be allowed to flow from the pressuretank via the ultrafiltration module to the drain. The sterile water fromthe pressure tank (vessel for filtered water) will flow in the oppositedirection compared with the direction of flow during filtration, throughthe ultrafiltration membrane, thereby cleaning its pore structure aswell as the module housing. In this way contaminants retained by themembrane are flushed away.

The post-treatment filter is provided after the ultrafiltration membraneand the vessel for filtered water, but before the outlet. The filterconsists of, for example, a cylindrical block of activated carbon, whichmay be extruded or sintered. A microfiltration module is disposed in thecentral passage through the block, extending parallel to thelongitudinal axis. The water flows from the pressure tank through theblock of activated carbon and subsequently through the microfiltrationmembranes. Any harmful organic compounds possibly still present in thewater will be adsorbed on the activated carbon thereby allowing thetaste to be further improved. The micro filtration membrane module atthe inside of the block of activated carbon provides an effectivebarrier against back-growth of bacteria from the outlet into thepressure tank. The combination of ultrafiltration before the pressuretank, micro filtration module after the pressure tank, as well asregular washing of the ultrafiltration membrane module guarantees thatthe sterile water in the pressure tank is protected very effectivelyagainst bacterial contamination. In this way a very sterile system isprovided.

The ultrafiltration membrane module may also be formed by a cylindricalblock, which around its circumference is provided with openings for theinlet of water to be filtered. In this cylindrical block the filtrationmembranes may be provided, analogous to the first filter.

Below follows a description of the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagram of the filtering system according to theinvention.

-   -   FIG. 2 shows a 3-way valve in a position to drain flush water        from a membrane of the system to a drain.

The figure shows a housing 1 provided with an activated carbon firstfilter module 2 through which the water to be filtered is fed via aninlet 11. Such modules are known in practice. The pre-filtered waterleaves this first housing 1 and is fed via a three-way valve to thehousing 4 of the ultrafiltration membrane module 5. In the embodimentshown, the three-way valve 3 is in the first position, allowing thepre-filtered water from of the first filter housing 1 to be fed to thesecond housing 4. The housing 4 comprises the ultrafiltration membranemodule 5. This ultrafiltration membrane module 5 is cylindrical. Such anembodiment is also known in the art. Subsequently the water from theultrafiltration membrane module is fed via the outlet 12 to a vessel 6for filtered water. This vessel 6 is embodied as a pressure tank. Fromthe vessel 6 the filtered water can be fed via a post-treatment filter14 in housing 7 to a faucet 9. The post-treatment filter 14 ispreferably comprised of a cylindrical carbon filter having a coaxialcavity 15, in which coaxial cavity 15 a microfiltration membrane 13 isprovided. This microfiltration module is preferably comprised of abundle of hydrophilic membranes having an asymmetrical pore structure,with the filtering layer at the inside of the capillary, having a poresize of approximately 0.1 to approximately 0.4 μm.

Because there is a microfiltration membrane 13 between the faucet 9 andthe storage vessel 6 for filtered water, no back-growth of contaminantsvia the faucet 9 to the storage vessel 6 is possible. This guaranteesthe sterility of the filtered water in the vessel 6.

When the ultrafiltration membrane 5 becomes contaminated due to theretention of substances that have been removed from the water to befiltered, filtered water can be flushed back from the vessel 6 throughthe ultrafiltration membrane and subsequently to a drain 10. This drain10 can be reached by setting the three-way valve 3 into a secondposition as shown in FIG. 2. When flushing back, the faucet 9 must ofcourse be closed. By positioning the three-way valve 3 as near aspossible to the feed side 16 of the housing 4 of the ultrafiltrationmodule 5, and at the same time keeping the distance from the outlet 12to the vessel 6 for filtered water as short as possible, only verylittle water will be required for cleaning the ultrafiltration membranemodule 5 in accordance with the procedure described above.

The above-described sequence of separate filtering means, three-wayvalve and storage vessel, between the inlet and the outlet of the systemprovides a very convenient system for the purification of drinkingwater.

The invention is of course not limited to the illustrations in thefigures and the above description. A person skilled in the art will knowof further adaptations.

1. A filtering system comprising: an inlet for liquid to be filtered, anoutlet for filtered liquid, and filtering means, placed as modules inseries, the filtering means comprising: a first filter comprisingactivated carbon to provide a primary filtration of up to approximately0.5 μm, an ultrafiltration membrane having a pore size of approximately0.02 μm, a valve disposed between the first filter and theultrafiltration membrane, or disposed between housings in which thefirst filter and the ultrafiltration membrane are accommodated, thevalve in a first position connecting the first filter with theultrafiltration membrane, and in a second position connecting theultrafiltration membrane with a drain for discharging contaminant fromthe ultrafiltration membrane via said valve; a post-treatment filtercomprising activated carbon with a microfiltration module that isconnected to the outlet; and a storage vessel for filtered waterdisposed between the ultrafiltration membrane and the post-treatmentfilter, said filtered water backwashing the ultrafiltration membranewhen the valve is in the second position.
 2. A system according to claim1, wherein each of the first filter, the ultrafiltration membrane andthe post-treatment filter is accommodated in a separate housing.
 3. Asystem according to claim 1, wherein the ultrafiltration membranecomprises a dead-end configuration.
 4. A system according to claim 1,wherein the ultrafiltration membrane comprises a dead-end flushconfiguration.
 5. A system according to claim 1, wherein theultrafiltration membrane comprises hydrophilic capillary membraneshaving an asymmetrical pore structure with a filtering layer on theinside of the capillary membranes.
 6. A system according to claim 1,wherein the post-treatment filter comprises a cylindrical block ofactivated carbon having a substantially coaxial open cavity, themicrofiltration module being provided in the open cavity.
 7. A systemaccording to claim 6, wherein the microfiltration module compriseshydrophilic capillary membranes having an asymmetrical pore structurewith a filtering layer on the inside of the capillary membranes.
 8. Asystem according to claim 1, wherein the valve is a three-way valve. 9.A method of operating a filtering system comprising an inlet for liquidto be filtered, an outlet for filtered liquid, and filtering means,placed as modules in series, the filtering means comprising a firstfilter comprising activated carbon to provide a primary filtration of upto approximately 0.5 μm, an ultrafiltration membrane having a pore sizeof approximately 0.02 μm, a valve disposed between the first filter andthe ultrafiltration membrane, or disposed between housings in which thefirst filter and the ultrafiltration membrane are accommodated, thevalve in a first position connecting the first filter with theultrafiltration membrane, and in a second position connecting theultrafiltration membrane with a drain, a post-treatment filtercomprising activated carbon with a microfiltration module that isconnected to the outlet; and a storage vessel for filtered waterdisposed between the ultrafiltration membrane and the post-treatmentfilter, the method comprising: closing the outlet; conducting, incounterflow, filtered water from the storage vessel with the valve inthe second position to remove contamination from the membrane; andsubsequently draining water with the contamination from theultrafiltration membrane via the valve through the drain.
 10. The methodaccording to claim 9, comprising: passing water through a first filterof activated carbon to provide a primary filtration of up toapproximately 0.5 μm, then through an ultrafiltration membrane modulehaving a pore size of approximately 0.02 μm , then through apost-treatment filter comprising activated carbon; and discharging thethus filtered water via an outlet.
 11. The method according to claim 10,comprising collecting the water in a storage vessel after the waterpasses through the ultrafiltration membrane module.
 12. The methodaccording to claim 11, comprising passing the water from the storagevessel through the post-treatment filter comprised of activated carbonto the outlet.
 13. The method according to claim 10, comprising passingthe water through the ultrafiltration module at a rate of between 0.2 to20 liters per minute.
 14. The method according to claim 9, wherein thevalve is a three-way valve.